Photovoltaic device

ABSTRACT

Methods and apparatus are provided for converting electromagnetic radiation, such as solar energy, into electric energy with increased efficiency when compared to conventional solar cells. A photovoltaic (PV) unit, according to embodiments of the invention, may have a very thin absorber layer produced by epitaxial lift-off (ELO), all electrical contacts positioned on the back side of the PV device to avoid shadowing, and/or front side and back side light trapping employing a diffuser and a reflector to increase absorption of the photons impinging on the front side of the PV unit. Several PV units may be combined into PV banks, and an array of PV banks may be connected to form a PV module with thin strips of metal or conductive polymer applied at low temperature. Such innovations may allow for greater efficiency and flexibility in PV devices when compared to conventional solar cells.

CROSS-REFERENCE TO RELATED APPLICATIONS

Under 35 U.S.C. §120, this application is a divisional application andclaims the benefit of priority to U.S. patent application Ser. No.12/605,108, filed Oct. 23, 2009 and U.S. Provisional Patent ApplicationSer. No. 61/107,954, filed Oct. 23, 2008, all of which is incorporatedherein by reference.

BACKGROUND

1. Technical Field

Embodiments of the present invention generally relate to photovoltaic(PV) devices, such as solar cells, with increased efficiency and greaterflexibility and methods for fabricating the same.

2. Description of the Related Art

As fossil fuels are being depleted at ever-increasing rates, the needfor alternative energy sources is becoming more and more apparent.Energy derived from wind, from the sun, and from flowing water offerrenewable, environment-friendly alternatives to fossil fuels, such ascoal, oil, and natural gas. Being readily available almost anywhere onEarth, solar energy may someday be a viable alternative.

To harness energy from the sun, the junction of a solar cell absorbsphotons to produce electron-hole pairs, which are separated by theinternal electric field of the junction to generate a voltage, therebyconverting light energy to electric energy. The generated voltage can beincreased by connecting solar cells in series, and the current may beincreased by connecting solar cells in parallel. Solar cells may begrouped together on solar panels. An inverter may be coupled to severalsolar panels to convert DC power to AC power.

Nevertheless, the currently high cost of producing solar cells relativeto the low efficiency levels of contemporary devices is preventing solarcells from becoming a mainstream energy source and limiting theapplications to which solar cells may be suited. Accordingly, there is aneed for more efficient photovoltaic devices suitable for a myriad ofapplications.

SUMMARY OF THE INVENTION

Embodiments of the present invention generally relate to methods andapparatus for converting electromagnetic radiation, such as solarenergy, into electric energy with increased efficiency when compared toconventional solar cells.

One embodiment of the present invention provides a photovoltaic (PV)device. The PV device generally includes a p⁺-doped layer, an n-dopedlayer disposed above the p⁺-doped layer to form a p-n layer such thatelectric energy is created when light is absorbed by the p-n layer, awindow layer disposed above the n-doped layer, an antireflective coatingdisposed above the window layer, and a diffuser disposed below thep⁺-doped layer.

BRIEF DESCRIPTION OF THE DRAWINGS

So that the manner in which the above-recited features of the inventioncan be understood in detail, a more particular description of theinvention, briefly summarized above, may be had by reference toembodiments, some of which are illustrated in the appended drawings. Itis to be noted, however, that the appended drawings illustrate onlytypical embodiments of this invention and are therefore not to beconsidered limiting of its scope, for the invention may admit to otherequally effective embodiments.

FIG. 1 illustrates multiple epitaxial layers for a photovoltaic (PV)unit in cross-section with example thickness, composition, and doping ofthe semiconductor layers, in accordance with an embodiment of thepresent invention.

FIGS. 2A-D illustrate various layer stack profiles for the base andemitter layers of the PV unit, in accordance with embodiments of thepresent invention.

FIGS. 3A and 3B illustrate semiconductor layers for a PV unit withoffset p-n layers between the base and emitter layers, in accordancewith embodiments of the present invention.

FIG. 4 illustrates semiconductor layers for a PV unit with an emitterlayer having a doping profile fine-tuned such that the doping levelsincrease from the p-n layer to the top of the emitter layer, inaccordance with an embodiment of the present invention.

FIG. 5 illustrates semiconductor layers for a PV unit with multipleAlGaAs emitter layers having graded aluminum (Al) levels, in accordancewith an embodiment of the present invention.

FIG. 6 illustrates contacts to the semiconductor layers being on theback side of the PV unit, in accordance with an embodiment of thepresent invention.

FIG. 7 illustrates passivation on the edges of the recesses in theemitter layer, in accordance with an embodiment of the presentinvention.

FIG. 8 illustrates an antireflective coating added to the semiconductorlayers on the front side of the PV unit, in accordance with anembodiment of the present invention.

FIG. 9 illustrates roughening a window layer before applying theantireflective coating, in accordance with an embodiment of the presentinvention.

FIG. 10 illustrates multiple window layers, wherein the outermost windowlayer is roughened before the antireflective coating is applied, inaccordance with an embodiment of the present invention.

FIG. 11 illustrates a roughened emitter layer on the back side of the PVunit, in accordance with an embodiment of the present invention.

FIG. 12 illustrates a diffuser on the back side of the PV unit, inaccordance with an embodiment of the present invention.

FIG. 13 illustrates dielectric particles and white paint functioning asthe diffuser of FIG. 12, in accordance with an embodiment of the presentinvention.

FIG. 14 illustrates metal particles functioning as the diffuser of FIG.12, in accordance with an embodiment of the present invention.

FIG. 15A illustrates the back side of the PV unit, in accordance with anembodiment of the present invention.

FIG. 15B illustrates an equivalent electrical circuit of the PV unit ofFIG. 15A, in accordance with an embodiment of the present invention.

FIG. 16 illustrates the interconnection of the p-contacts and of then-contacts between the multiple PV units to form a PV bank, inaccordance with an embodiment of the present invention.

FIG. 17 illustrates the interconnection of multiple PV banks to form aPV module, in accordance with an embodiment of the present invention.

DETAILED DESCRIPTION

Embodiments of the present invention provide techniques and apparatusfor converting electromagnetic radiation, such as solar energy, intoelectric energy with increased efficiency when compared to conventionalsolar cells.

An Exemplary Thin Absorber Layer

FIG. 1 illustrates various epitaxial layers of a photovoltaic (PV) unit100 in cross-section during fabrication. The various layers may beformed using any suitable method for semiconductor growth, such asmolecular beam epitaxy (MBE) or metalorganic chemical vapor deposition(MOCVD), on a substrate (not shown).

To form the PV unit 100, one or more buffer layers may be formed on thesubstrate. The purpose of the buffer layer(s) is to provide anintermediary between the substrate and the semiconductor layers of thefinal PV unit that can accommodate their different crystallographicstructures as the various epitaxial layers are formed. Having athickness of about 200 nm, for example, a buffer layer 102 may comprisea group III-V compound semiconductor, such as gallium arsenide (GaAs),depending on the desired composition of the final PV unit. For someembodiments, for example, the substrate may comprise GaAs when creatinga GaAs buffer layer.

For some embodiments, a release layer 104 may be formed above the bufferlayer 102. The release layer 104 may comprise aluminum arsenide (AlAs),for example, and have a thickness in a range from about 5 to 10 nm. Thepurpose of the thin release layer 104 is described in greater detailbelow.

Above the release layer 104, a window layer 106 may be formed. Thewindow layer 106 may comprise aluminum gallium arsenide (AlGaAs), suchas Al_(0.3)Ga_(0.7)As. The window layer 106 may have a thickness in arange of about 5 to 30 nm (e.g., 20 nm as shown) and may be undoped. Thewindow layer 106 may be transparent to allow photons to pass through thewindow layer on the front side of the PV unit to other underlyinglayers.

A base layer 108 may be formed above the window layer 106. The baselayer 108 may comprise any suitable group III-V compound semiconductor,such as GaAs. The base layer 108 may be monocrystalline. The base layer108 may be n-doped, and for some embodiments, the doping concentrationof the n-doped base layer 108 may be in a range from about 1×10¹⁶ to1×10¹⁹ cm⁻³ (e.g., 2×10¹⁷ cm⁻³ as shown). The thickness of the baselayer 108 may be in a range from about 300 to 3500 nm.

As illustrated in FIG. 1, an emitter layer 110 may be formed above thebase layer 108. The emitter layer 110 may comprise any suitable groupIII-V compound semiconductor for forming a heterojunction with the baselayer 108. For example, if the base layer 108 comprises GaAs, theemitter layer 110 may comprise a different semiconductor material, suchas AlGaAs. If the emitter layer 110 and the window layer 106 bothcomprise AlGaAs, the Al_(x)Ga_(1-x)As composition of the emitter layer110 may be the same as or different than the Al_(y)Ga_(1-y)Ascomposition of the window layer 106. The emitter layer 110 may bemonocrystalline. The emitter layer 110 may be heavily p-doped (i.e.,p⁺-doped), and for some embodiments, the doping concentration of thep⁺-doped emitter layer may be in a range from about 1×10¹⁷ to 1×10²⁰cm⁻³ (e.g., 1×10¹⁹ cm⁻³ as shown). The thickness of the emitter layer110 may be about 300 nm, for example. The combination of the base layer108 and the emitter layer 110 may form an absorber layer for absorbingphotons. For some embodiments, the absorber layer may have a thicknessless than 800 nm, or even less than 500 nm.

The contact of an n-doped base layer to a p⁺-doped emitter layer createsa p-n layer 112. When light is absorbed near the p-n layer 112 toproduce electron-hole pairs, the built-in electric field may force theholes to the p⁺-doped side and the electrons to the n-doped side. Thisdisplacement of free charges results in a voltage difference between thetwo layers 108, 110 such that electron current may flow when a load isconnected across terminals coupled to these layers.

Rather than an n-doped base layer 108 and a p⁺-doped emitter layer 110as described above, conventional photovoltaic semiconductor devicestypically have a p-doped base layer and an n⁺-doped emitter layer. Thebase layer is typically p-doped in conventional devices due to thediffusion length of the carriers. Fabricating a thinner base layeraccording to embodiments of the invention allows for the change to ann-doped base layer. The higher mobility of electrons in an n-doped layercompared to the mobility of holes in a p-doped layer leads to the lowerdoping density in the n-doped base layer 108 of embodiments of theinvention.

Once the emitter layer 110 has been formed, cavities or recesses 114 maybe formed in the emitter layer deep enough to reach the underlying baselayer 108. Such recesses 114 may be formed by applying a mask to theemitter layer 110 using photolithography, for example, and removing thesemiconductor material in the emitter layer 110 not covered by the maskusing any suitable technique, such as wet or dry etching. In thismanner, the base layer 108 may be accessed via the back side of the PVunit 100.

For some embodiments, an interface layer 116 may be formed above theemitter layer 110. The interface layer 116 may comprise any suitablegroup III-V compound semiconductor, such as GaAs. The interface layer116 may be p⁺-doped, and for some embodiments, the doping concentrationof the p⁺-doped interface layer 116 may be 1×10¹⁹ cm⁻³. The thickness ofthe interface layer 116 may be about 300 nm, for example.

Once the remaining epitaxial layers have been formed above the releaselayer 104, the thin release layer 104 may be sacrificed via etching withaqueous HF, for example. In this manner, the functional layers of the PVunit 100 (e.g., the window layer 106, the base layer 108, and theemitter layer 110) may be separated from the buffer layer(s) 102 andsubstrate during the epitaxial lift-off (ELO) process.

A PV unit created in this manner has a significantly thin absorber layer(e.g., <500 nm) compared to conventional solar units, which may beseveral micrometers thick. The thickness of the absorber layer isproportional to dark current levels in the PV unit (i.e., the thinnerthe absorber layer, the lower the dark current). Dark current is thesmall electric current that flows through the PV unit or other similarphotosensitive device (e.g., a photodiode) even when no photons areentering the device. This background current may be present as theresult of thermionic emission or other effects. Because the open circuitvoltage (V_(oc)) increases as the dark current is decreased in aphotosensitive semiconductor device, a thinner absorber layer may mostlikely lead to a greater V_(oc) for a given light intensity and, thus,increased efficiency. As long as the absorber layer is able to traplight, the efficiency increases as the thickness of the absorber layeris decreased.

The thinness of the absorber layer may not only be limited by thecapabilities of thin film technology and ELO. For example, efficiencyincreases with the thinness of the absorber layer, but the absorberlayer should be thick enough to carry current. However, higher dopinglevels may allow current to flow, even in very thin absorber layers.Therefore, increased doping may be utilized to fabricate very thinabsorber layers with even greater efficiency. Conventional PV devicesmay suffer from volume recombination effects, and therefore, suchconventional devices do not employ high doping in the absorber layer.The sheet resistance of the absorber layer may also be taken intoconsideration when determining the appropriate thickness.

Not only does a thin absorber layer lead to increased efficiency, but PVunits with such a thin absorber layer may be more flexible thanconventional solar cells having a thickness of several micrometers.Therefore, PV units according to embodiments of the invention may beappropriate for a greater number of applications than conventional solarcells.

FIGS. 2A-D illustrate various layer stack profiles 200 _(a-d) for thebase and emitter layers 108, 110 of the PV unit, in accordance withembodiments of the present invention. The layer stack profile 200 _(a)in FIG. 2A illustrates the base and emitter layers 108, 110 asillustrated in FIG. 1. For some embodiments, an intermediate layer 202may be formed above the base layer 108, and the emitter layer 110 may beformed above the intermediate layer. The intermediate layer 202 mayprovide a more gradual transition between the base and emitter layers108, 110.

The intermediate layer 202 may be n-doped, heavily n-doped (i.e.,n⁺-doped), or p⁺-doped. For example, FIG. 2B illustrates an intermediatelayer 202 _(b) comprising n-AlGaAs. As another example, FIG. 2C depictsan intermediate layer 202 _(c) comprising n⁺-AlGaAs. As yet anotherexample, FIG. 2D portrays an intermediate layer 202 _(d) comprisingp⁺-GaAs.

In FIG. 1, the p-n layer 112 between the base layer 108 and the emitterlayer 110 is flat and is not exposed in the recesses 114. In otherwords, the p-n layer 112 of FIG. 1 may be considered as a plane havingonly two-dimensional geometry. For some embodiments, as shown in FIGS.3A and 3B, the semiconductor layers for a PV unit may be formed tocreate an offset p-n layer 312 between the base and emitter layers 108,110. In other words, an offset p-n layer 312 may be considered to havethree-dimensional geometry. An offset p-n layer 312 may be exposed inthe recesses 114.

As illustrated in FIG. 3A, an offset p-n layer 312 _(a) may be producedby removing semiconductor material all the way through the emitter layer110 and partially into the base layer 108 when forming the recesses 114as described above. Another method of forming an offset p-n layer 312_(b), as illustrated in FIG. 3B, may comprise applying a mask to thebase layer 108 before forming the emitter layer 110. Semiconductormaterial may be removed via any suitable technique, such as etching,from a portion of the base layer 108 where the emitter layer is intendedto remain (i.e., everywhere except the desired locations of the recesses114). Once the emitter layer 110 and the recesses 114 are formed in theemitter layer, the resulting offset p-n layer 312 _(b) has a greatersurface area than a flat p-n layer 112.

For some embodiments, doping levels may be fine-tuned within a layer ofthe PV unit during fabrication. For example, FIG. 4 illustrates a PVunit 400 with an emitter layer 110 having a doping profile fine-tunedsuch that the doping concentration increases from the p-n layer 112 tothe top of the emitter layer 110 in the z-direction.

For some embodiments, the emitter layer 110 may comprise multiplelayers, and the multiple layers may comprise different compositions. Forexample, FIG. 5 illustrates semiconductor layers for a PV unit 500 withmultiple p⁺-AlGaAs emitter layers having graded aluminum (Al) levels(i.e., percentages), in accordance with an embodiment of the presentinvention. In this example embodiment, a first emitter layer 510 ₁comprising p⁺-GaAs without any aluminum may be formed above the baselayer 108. A second emitter layer 510 ₂ comprising p⁺-Al_(0.1)Ga_(0.3)Asmay be formed above the first emitter layer 510 ₁. Then, a third emitterlayer 510 ₃ comprising p⁺-Al_(0.2)Ga_(0.8)As and a fourth emitter layer510 ₄ comprising p⁺-Al_(0.3)Ga_(0.7)As may be formed above the secondemitter layer 510 ₂, in turn. Having such graded Al levels may avoidjunction barriers.

Exemplary Electrical Contacts

Electrical contacts may be used to couple the semiconductor layers ofthe PV unit 100 to wires for connection to other PV units and forexternal connection to a load. A conventional solar cell typically hascontacts on both the front and back sides of the cell. Front sidecontacts, especially thicker ones, create shadows where light cannotreach the underlying absorber layer to be converted into electricenergy. Therefore, the efficiency potential of the solar cell cannot beobtained. Accordingly, techniques and apparatus for contacting thesemiconductor layers of the PV unit without introducing shadows areneeded.

FIG. 6 illustrates all electrical contacts to the semiconductor layersbeing on the back side of the PV unit 100, according to an embodiment ofthe present invention. For example, n-contacts 602 may be formed in therecesses 114 to provide an interface to the n-doped base layer 108, andp-contacts 604 may be formed above the interface layer 116 to couple tothe p⁺-doped emitter layer 110. The heavy doping in the p⁺-dopedinterface layer 116 may facilitate making an ohmic contact. In thismanner, efficiency need not be sacrificed by having electrical contactson the front side of the PV unit to block light and create solarshadows.

The pattern of the recesses 114 in the emitter layer 110 and theremaining portion of the interface layer 116 for the contacts 602, 604may be based on the desired sheet resistance. The dimensions (e.g.,area) of the contacts 602, 604 may be very small compared to thedimensions (e.g., area) of a single PV unit 100. What is more, thepattern of the contacts 602, 604 may provide a built-in toleranceagainst local defects and shadowing.

The contacts 602, 604 may comprise any suitable electrically conductivematerial, such as a metal or a metal alloy. Preferably, the material forthe contacts should not punch through the semiconductor layers duringfabrication. Traditional contacts comprising gold (Au) often had thisspiking problem. Furthermore, the material for the back side contactsmay preferably be capable of being applied at relatively lowmetallization process temperatures, such as between 150 and 200° C. Forexample, the contact 602, 604 may comprise palladium/germanium (Pd/Ge)to meet these design goals. Palladium does not react with GaAs.

Whatever material is selected, the contacts 602, 604 may be fabricatedon the PV unit 100 by any suitable method, such as vacuum-evaporationthrough a photoresist, photolithography, screen printing, or merelydepositing on the exposed portion of the PV units that have beenpartially covered with wax or another protective material. These methodsall involve a system in which the part of the PV unit on which a contactis not desired is protected, while the rest of the PV unit is exposed tothe metal. Of these, screen printing may be the most cost effective,helping to decrease the cost of the resulting PV devices.

Despite all the contacts 602, 604 being on the back side of the PV unit100 to reduce solar shadows, dark current and its stability with timeand temperature may still be concerns when designing an efficient PVunit. An exposed p-n layer 112 may be a source of dark current, andlarger recesses 114 may be responsible for an increase in dark current.Thus, smaller recesses 114 may be desired. However, there is a tradeoffbetween reducing the size of the recesses 14 to reduce dark current andthe probability of fabricating the n-contacts 602 in the recesses 114without touching the sidewalls.

Therefore, for some embodiments, the sidewalls of the recesses 114 maybe passivated as another way to reduce the dark current in the PV unit.FIG. 7 illustrates passivation 702 on the sidewalls (i.e., lateralsurfaces) of the recesses 114 in the emitter layer 110, in accordancewith an embodiment of the present invention. The sidewalls may bepassivated most likely before—but possibly after—the n-contacts 602 areformed, using any suitable passivation method, such as chemical vapordeposition (CVD) or plasma-enhanced CVD (PECVD). The passivation 702 maycomprise any suitable electrically non-conductive material, such assilicon nitride (SiN), SiO_(x), TiO_(x), TaO_(x), zinc sulfide (ZnS), orany combination thereof.

Exemplary Light Trapping

To achieve efficiency, the absorber layer of an ideal photovoltaic (PV)device would absorb all of the photons impinging on the PV device'sfront side facing the light source since the open circuit voltage(V_(oc)) or short circuit current (I_(sc)) is proportional to the lightintensity. However, several loss mechanisms typically interfere with thePV device's absorber layer seeing or absorbing all of the light reachingthe front side of the device. For example, the semiconductor layers ofthe PV device may be shiny (especially when made of pure silicon) and,therefore, may reflect a substantial portion of the impinging photons,preventing these photons from ever reaching the absorber layer. If twosemiconductor layers (e.g., the window layer and the base layer) have adifferent index of refraction, some of the photons reaching theinterface between these two layers may be reflected according to Snell'sLaw if their angle of incidence is too high, again preventing thesephotons from reaching the absorber layer. Furthermore, the absorberlayer may not absorb all of the impinging photons; some photons may passthrough the absorber layer without affecting any electron-hole pairs.

Accordingly, there is a need for techniques and apparatus to capture thelight impinging on the front side of the PV device such that as manyphotons as possible may be absorbed by the absorber layer and convertedinto electric energy. In this manner, the PV device's efficiency may beincreased.

Apparatus for trapping the light within the semiconductor layers of a PVdevice may be divided into two categories: front side light trapping andback side light trapping. By employing both types of light trapping in aPV device, the idea is that nearly all photons impinging on the PVdevice's front side may be captured and “bounce around” within thesemiconductor layers until the photons are absorbed by the absorberlayer and converted to electric energy.

Exemplary Front Side Light Trapping

FIG. 8 illustrates an antireflective (AR) coating 802 disposed adjacentto the window layer 106 on the front side of the PV unit 100, inaccordance with an embodiment of the present invention. According to itspurpose, the AR coating 802 may comprise any suitable material thatallows light to pass through while preventing light reflection from itssurface. For example, the AR coating 802 may comprise magnesium fluoride(MgF₂), zinc sulfide (ZnS), silicon nitride (SiN), titanium dioxide(TiO₂), silicon dioxide (SiO₂), or any combination thereof. The ARcoating 802 may be applied to the window layer 106 by any suitabletechnique, such as sputtering.

For some embodiments, the window layer 106 may be roughened or texturedbefore applying the antireflective coating 802. FIG. 9 illustrates aroughened window layer 106. Roughening of the window layer 106 may beaccomplished by wet etching or dry etching, for example. Texturing maybe achieved by applying small particles, such as polystyrene spheres, tothe surface of the window layer 106 before applying the AR coating 802.By roughening or texturing the window layer 106, different angles areprovided at the interface between the AR coating 802 and the windowlayer, which may have different indices of refraction. In this manner,more of the incident photons may be transmitted into the window layer106 rather than reflected from the interface between the AR coating 802and the window layer because some photons' angles of incidence are toohigh according to Snell's Law. Thus, roughening or texturing the windowlayer 106 may provide increased light trapping.

Also for some embodiments, the window layer 106 may comprise multiplewindow layers. For these embodiments, the outermost window layer (i.e.,the window layer closest to the front side of the PV unit 100) may beroughened or textured as described above before the antireflectivecoating 802 is applied, as illustrated in FIG. 10. In FIG. 10, thewindow layer 106 comprises a first window layer 1002 disposed adjacentto the base layer 108 and a second window layer 1004 interposed betweenthe first window layer 1002 and the antireflective coating 802. Thefirst and second window layers 1002, 1004 may comprise any materialsuitable for the window layer 106 as described above, such as AlGaAs,but typically with different compositions. For example, the first windowlayer 1002 may comprise Al_(0.3)Ga_(0.7)As, and the second window layer1004 may comprise Al_(0.1)Ga_(0.9)As. Furthermore, some of the multiplewindow layers may be doped, while others are undoped for someembodiments. For example, the first window layer 1002 may be doped, andthe second window layer 1004 may be undoped.

Exemplary Back Side Light Trapping

For some embodiments, the emitter layer 110 on the back side of the PVunit 100 may be roughened or textured, as described above with respectto the front side, in an effort to increase light trapping. FIG. 11illustrates such a roughened emitter layer 110.

FIG. 12 illustrates a diffuser 1202 on the back side of the PV unit 100in an effort to increase the amount of light captured by the absorberlayer. Rather than reflecting photons similar to a mirror where theangle of reflectance equals the angle of incidence, the purpose of thediffuser 1202 is to diffuse or scatter photons that pass through theabsorber layer without being absorbed. For some embodiments, thediffuser 1202 may be covered with a reflective layer 1204. In thismanner, the diffuser 1202 may provide new angles to incident photons,some of which may be redirected back to the interior of the PV unit. Forother photons that are directed to the back side of the PV unit, thereflective layer 1204 may redirect these photons back through thediffuser 1202 and towards the interior of the PV unit. Although some ofthe light may be absorbed by the diffuser 1202 as the photons arescattered and redirected inside, much of the light is redirected to theabsorber layer to be absorbed and converted into electric energy,thereby increasing efficiency. Conventional PV devices without adiffuser and a reflective layer may not be able to recapture photonsthat reach the back side of the device without being absorbed initiallyby the absorber layer.

For some embodiments, the diffuser 1202 may comprise dielectricparticles 1302, as illustrated in FIG. 13. The dielectric particles maycomprise any suitable material which is electrically insulative and doesnot absorb light. The dielectric particles 1302 may have a diameter inrange from about 0.2 to 2.0 μm. The dielectric particles 1302 may becovered by white paint 1304, which reflects light and may act as thereflective layer for redirecting photons back to the interior of the PVunit 100. The white paint 1304 may comprise TiO₂, for example.

For some embodiments, the diffuser 1202 may comprise metal particles1402, as illustrated in FIG. 14. The metal particles 1402 may reflectphotons that were not absorbed by the absorber layer, and by having amultitude of metal particles 1402, the photons may be scattered indifferent directions several times before being redirected to theinterior of the PV unit 100. The metal particles 1402 may have adiameter of about 150 to 200 nm, functioning as relatively compactscatterers. With thinner particles in the diffuser 1202, the thicknessof the PV unit 100 may be kept smaller, thereby maintaining the desiredflexibility of the PV unit 100.

Because the metal particles 1402 are electrically conductive, lateralsurfaces of the interface layer 116 may be passivated to prevent themetal particles 1402 from interfering with the operation of the device.The interface layer 116 may be passivated using any suitable passivationmethod, such as chemical vapor deposition (CVD) or plasma-enhanced CVD(PECVD). The passivation 1404 may comprise any suitable electricallynon-conductive material, such as silicon nitride (SiN), SiO_(x),TiO_(x), TaO_(x), zinc sulfide (ZnS), or any combination thereof.Furthermore, for some embodiments, a dielectric layer 1406 may be formedabove the metal particles 1402 in an effort to avoid shunting thecontacts 602, 604, as depicted in FIG. 14. The dielectric layer 1406 maycomprise any suitable electrically insulative material, such as SiO₂,SiN, or glass.

Exemplary Integration

FIG. 15A illustrates the back side of the PV unit 100, wherein all thecontacts 602, 604 are disposed on the back side. As described above, then-contacts 602 may be located within the recesses 114 in the emitterlayer 110. The PV unit 100 may have a width w of about 2 to 3 cm and alength l of about 10 cm.

FIG. 15B illustrates an equivalent electrical circuit 1500 of the PVunit 100 of FIG. 15A. One may consider the PV unit 100 as having anefficient miniature solar cell 1502 between each n-contact 602 andp-contact 604. Within a PV unit 100, all of the n-contacts 602 arecoupled to the same base layer 108 and all of the p-contacts 604 arecoupled to the same emitter layer 110. Therefore, the open circuitvoltage (V_(oc)) of the equivalent circuit 1500 may be modeled as thesum of the open circuit voltages across the miniature solar cells 1502in series, and the short circuit current (I_(sc)) may be modeled as thesum of the short circuit currents across the miniature solar cells 1502in parallel. In essence, the equivalent electrical circuit 1500 of thePV unit 100 may be thought of as a single solar cell with a greaterV_(oc) and a larger I_(sc) than those of the miniature solar cells 1502which compose it.

FIG. 16 illustrates the interconnection of the p-contacts 604 and of then-contacts between multiple PV units 100 to form a PV bank 1600, inaccordance with an embodiment of the present invention. For someembodiments, a PV bank 1600 may comprise one column of about ten PVunits 100 arranged in parallel. In this manner, the short circuitcurrent (I_(sc)) of the PV bank 1600 may be about ten times greater thanthat of a single PV unit 100.

The interconnection may be accomplished by thin strips 1602, 1604 placedbetween the PV units 100 according to a certain pattern. For example,strips 1604 may connect the p-contacts 604 of a first PV unit 100 ₁ tothe p-contacts 604 of a second PV unit 100 ₂. Rather than connecting thefirst PV unit 100 ₁ to the second PV unit 100 ₂ for the n-contacts,strips 1602 may connect the n-contacts 602 of the second PV unit 100 ₂to the n-contacts 602 of a third PV unit 100 ₃ as illustrated in FIG.16. This interconnection pattern may be selected to provide considerableflexibility in the PV bank 1600.

The strips 1602, 1604 may comprise any suitable electrically conductivematerial, such as metal or metal alloys. For example, the strips 1602,1604 may comprise tin-coated copper. For some embodiments, the strips1602 for the n-contacts may comprise a different material than thestrips 1604 for the p-contacts 604. To form strips made of metal ormetal alloys, the strips may be applied to the back side of the PV units100 across the “dot geometry” of the contacts 602, 604 via screenprinting, for example.

Screen printing metals or metal alloys may indicate a high processtemperature. Therefore, for some embodiments, the strips 1602, 1604 maycomprise an electrically conductive polymer instead of a metal or metalalloy. The conductive polymer strips may be formed by screen printing ata lower temperature than that suggested by silk-screening metal.

The spacing between adjacent PV units 100 may be about 1 to 2 mm on thePV bank 1600. This relatively close spacing may also allow for greaterflexibility in the PV bank 1600, especially when combined with aninterconnection pattern selected for this purpose, such as theinterconnection pattern described above.

FIG. 17 illustrates the interconnection of multiple PV banks 1600arranged in an array to form a PV module 1700, in accordance with anembodiment of the present invention. Adjacent PV banks 1600 in a row maybe connected together by couplings 1702. The couplings 1702 may connectthe n-contact 602 of one PV bank to the p-contact 604 of an adjacent PVbank within the row, such that the PV banks 1600 (and the equivalentelectric circuit) of a row are connected in series, thereby combiningthe open circuit voltage (V_(oc)) capabilities of the PV banks 1600. Thecouplings 1702 may comprise a wire or a strip of metal, metal alloy, ora conductive polymer, similar to the strips 1602, 1604 in the PV banks1600.

The couplings 1702 may also connect a p-contact 604 on each row of PVbanks 1600 to a p-side bus-bar 1704 on one side and connect an n-contact602 on each row of PV banks 1600 to an n-side bus-bar 1706 on the otherside of the PV module 1700. In this manner, the rows of series-connectedPV banks 1600 may be connected in parallel, thereby combining the shortcircuit current (I_(sc)) capabilities of the PV banks 1600. The bus-bars1704, 1706 may be relatively thick in an effort to carry substantialcurrent generated by the PV banks 1600 to a load (not shown). For someembodiments, the DC output voltage (V_(oc)) of the module 1700 may becoupled to an inverter in an effort to create AC voltage.

The finished PV module 1700 may be encapsulated. The front side of thePV module 1700 may be covered with a thin transparent sheet comprisingglass or plastic, for example. The length L of the module may be about 1m with a 4×4 array of PV banks 1600 as illustrated in FIG. 16.

By connecting the PV units 100 with the strips 1602, 1604 to form PVbanks 1600 and by integrating the PV banks 1600 with couplings 1702 todevelop the PV module 1700, the PV module 1700 may have a built-intolerance against local defects. In other words, a defect (e.g., a shuntbetween an n-contact 602 and a p-contact 604) localized to a PV unit 100need not cause the module 1700 to fail. Furthermore, protection may beadded at the macroscopic and/or the microscopic level. In other words,protection, such as fuses, may be added to one or more PV banks 1600and/or to the PV module 1700. For some embodiments, protection circuitrymay be built into the PV units 100 at the wafer level.

While the foregoing is directed to embodiments of the invention, otherand further embodiments of the invention may be devised withoutdeparting from the basic scope thereof, and the scope thereof isdetermined by the claims that follow.

The invention claimed is:
 1. A photovoltaic device, comprising: ap⁺-doped AlGaAs layer, wherein recesses are formed in the p⁺-dopedAlGaAs layer such that the recesses extend through the p⁺-doped AlGaAslayer; a n-doped GaAs layer directly adjacent to the p⁺-doped AlGaAslayer, wherein the n-doped GaAs layer and p⁺-doped AlGaAs layer form ap-n heterojunction such that electric energy is created when light isabsorbed by the p-n heterojunction; an interface layer comprising aGroup III-V compound semiconductor above the p⁺-doped AlGaAs layer; adiffuser above the p⁺-doped AlGaAs layer, wherein the diffuser layer iscovered with a reflective layer which provides for photons to beredirected through the diffuser layer and towards an interior of thephotovoltaic device, wherein the diffuser layer and the interface layerare in direct contact with the p⁺-doped AlGaAs layer, wherein thediffuser layer is found in direct contact with a first side of theinterface layer and in direct contact with a second side of theinterface layer, and wherein the recesses in the p⁺-doped AlGaAs layerextend through the diffuser layer and the reflective layer; a firstwindow layer below the n-doped GaAs layer; a second window layer belowthe first window layer, wherein the first window and second windowlayers comprise AlGaAs, but with different composition; and anantireflective coating disposed below the second window layer.
 2. Thephotovoltaic device of claim 1, wherein the first window layer is dopedand the second window layer is undoped.
 3. The photovoltaic device ofclaim 1, wherein a surface of the second window layer adjacent to theantireflective coating has been roughened to provide different anglesfor increased light trapping.
 4. The photovoltaic device of claim 1,wherein a bottom surface of the p⁺-doped AlGaAs layer has been roughenedto provide different angles for increased light trapping.
 5. Thephotovoltaic device of claim 1, wherein the diffuser comprises aplurality of dielectric particles.
 6. The photovoltaic device of claim1, wherein the diffuser comprises a plurality of metal particles.
 7. Thephotovoltaic device of claim 6, further comprising a dielectric layerdisposed below the plurality of metal particles.